During the previous year we have completed the measurement of the two-photon action cross section for Photofrin and have begun additional measurements for other photosensitizing compounds. Under typical imaging conditions, Photofrin is observed to change from one-photon to two-photon excitation as the excitation wavelength is varied from 700-750 nm. This has been verified by measuring the edge response from a solution of Photofrin dissolved in methanol as a function of wavelength. For wavelengths between 750 and 980 nm the two-photon action cross section decreases from approximately 0.6 GM to 0.2 GM. Both absolute and relative cross section measurements have been made and found to agree. The two-photon excitation cross section of other photosensitizers are at various stages of completion and described and accounted for elsewhere.